A process for the manufacture of resin-impregnated fiber-reinforced structural composite vessels and the product resulting therefrom. More particularly, pressure vessels are wrapped in stages with a resin-fibers matrix and are subjected to autofrettage pre-stressing technique to increase the vessel""s cycle life and burst strength.
It is known to wrap a variety of underlying shapes with fibers embedded in a matrix of resin so as to form fiber-reinforced plastic composite products, or FRP. The fiber acts as the structural portion wrapped over a normally weak shape such as a liner for a pressure vessel.
One such example is the manufacture of fiber-reinforced pressure vessels by wrapping hollow, substantially non-structural pressure vessel liners with fibers resulting in a vessel having burst pressure and cyclical loading capabilities beyond that of the liner alone. Fibers wrapped about the vessel liner act in tension when the vessel is pressurized.
Conventionally, a multiplicity of fibers spooled into tows are passed through heated resin baths containing catalyzed resins prior to being mechanically wrapped onto the vessel liner. The configuration of the winding is dependant upon the speed of rotation of the vessel liner and the rate of travel of the tow-dispensing apparatus. The most common configurations are helical, in which the tows are at a significant angle from the axis of the object being wrapped; circumferential, in which the tows are wound hoop-wise around the object; and polar, in which the tows are wrapped in the direction of the longitudinal axis of the object.
The resin is permitted to dry and is then cured. Curing relates to the process by which the resin is allowed to achieve its final chemical state and effect its purpose to provide reinforcement to the liner. Curing or chemical poly-condensation, is the formation of polymers from monomers with the release of water or another simple substance. Curing is usually performed at elevated temperatures however, room temperature may be sufficient for some types of resins.
In some cases the resulting vessel is pressed into service after curing.
In other known processes, the liner is pre-stressed using a technique called autofrettage wherein the liner is plastically deformed (yielded) within the fiber-wrap for imparting a compressive residual stress into the liner and thereby increasing the vessel""s structural characteristics under pressure.
In more detail, a liner forms a fluid nozzle structure for providing access to the liner""s bore. In the autofrettage process, the cured fiber-wrapped liner is placed inside a protective housing and liquid is injected through the fluid nozzle structure, over-pressurizing the liner within the wrap and causing it to yield. When the fluid pressure is released, the tension in the liner diminishes and then becomes compressive at ambient pressure (pre-stressed). Accordingly, as the pressure rises once again, in service, the stress on the liner progressively reduces from compressive, through zero, then finally becomes tensile at a peak stress less than that previously experienced at the same pressure. Cyclical tensile stress is a major factor in precipitating fatigue stress failures and thus, with an initial compressive stress, each pressure cycle results in a lower maximum tensile stress in the liner and thus improves its fatigue strength.
It is known to apply autofrettage pressures of 6000 psig to pre-stress aluminum-lined, fiber-wrapped tanks which are safely operable at service pressures of about 3000 psig.
However, increased use of alternative fuels to fuel vehicles, such as compressed natural gas and hydrogen, and the requirement for ever greater fuel range, has created a need for lightweight, safe tanks with even greater capacity. One method for avoiding increasing tank size and weight, new tanks must be capable of containing fuel at higher service pressures, upwards of 10,000 psig. It is possible to provide fiber-reinforced tanks capable of such pressures but without pre-stressing, the cyclical life is too short to be of commercial value. Accordingly a pre-stressing technique is suggested.
Using the prior art process, autofrettage pressures in the range of about 20,000 to 25,000 psig would be required in order to adequately yield an aluminum liner to produce a vessel capable of safely maintaining integrity at a cyclic service pressure of 10,000 psig. Unfortunately, at these high pressures the boss forming the nozzle and the internal threads used to attach various fittings to the tank liner, also yield. As the nozzle structure yields, its dimensions no longer accept conventionally sized valve bodies and fittings. The dimensional changes in this portion of the tank are not predictable, require close fitting tolerances and therefore one is unable to compensate for such deformation when originally sizing the nozzle structure. The deformation of the nozzle structure is related to the surface area of the nozzle, as provided by the nozzle""s port size.
One approach to minimize deformation and enable high autofrettage pressures would be to make the nozzle opening much smaller, thereby reducing the service area and lowering the stress. From a practical standpoint however, this does not solve the problem as the smaller openings would no longer be able to accept current valve bodies. Note that more and more components are integrated into the valve bodies, and therefore must be of sufficient size to incorporate components such as solenoid-controlled valves, manual override valves, pressure regulators and temperature control devices.
Further, attempts to thicken the walls or fiber-wrap the boss have only provided marginal increases in strength of the nozzle structure, insufficient to eliminate yield.
U.S. Pat. No. 4,571,969 to Tomita discloses an alternative to single high pressure autofrettage for steel liners operating at service pressures of about 100-200 ksig. Tomita suggests that application of a single pressure is generally disadvantageous in that it does not always produce the required residual tangential compressive stresses in alloy steel cylinder bores. Further, Tomita states that high pressure autofrettage deforms the bore of the cylinder so that it is no longer employable in its intended use, where bore size must be maintained within narrowly prescribed ranges. Tomita teaches a cyclical autofrettage technique applicable to alloy steel cylinders in which the vessel is subjected to multiple lower pressure cycles in order to avoid large diameter dilations and the associated tolerance problems. The rate of production would diminish and costs would escalate if each cylinder were to be cyclically pressurized until a suitable strength was achieved. Further, aluminum liners have a lower yield stress than alloy steel and therefore do not require the extreme pressures suggested by Tomita.
Thus, there remains a need for a process by which a fiber-reinforced pressure vessel can be produced; capable of maintaining its integrity at service pressures upwards of 10,000 psig, with a nozzle structure port and threads which can be reliably sized and retain their dimensions necessary to accept a variety of valve bodies and fittings. Ideally, the process should be accomplished with a minimum number of steps for efficient rates of production.
The current invention addresses the unsuitability of the existing single cycle autofrettage processes for manufacturing very high pressure reinforced-reinforced vessels for storing fuel gas. High pressure autofrettage is associated with disadvantages including imposing unpredictable and significant deformation of the bore of the vessels threaded nozzles. The novel process also uses a single pressure cycle, but it achieves the objectives of forming a residual compressive stress in the vessels liner using lower pressures, and standard autofrettage equipment by implementing a unique two stage resin-impregnated fiber wrapping process to produce pressure vessels capable of reliably withstanding high service pressures in the range of 10,000 psig.
In a broad aspect of the invention, process for manufacturing a fiber-reinforced high pressure vessel comprises the steps of:
wrapping the liner with a first composite layer of predetermined strength;
applying a predetermined first pressure to the bore, the first pressure being greater than the design pressure and sufficient to yield the liner within the first composite layer and produce residual compressive stress therein; and
wrapping the liner with a second composite layer so that when the design pressure is applied, the strain in the liner is constrained so that stress in the liner is below yield.
Preferably the first pressure and the strength of the first composite layer are complementary. A maximum pressure is set which would not damage the nozzle and further that the strength of the first composite layer is set such that the liner can be plastically deformed at a pressure less than that maximum pressure.
More preferably, the ductile metal liner is aluminum and the pressure used to autofrettage the liner is in the order of 15,000 psig, resulting in a product pressure vessel which, in combination with the strength provided by the second composite layer, is capable of cyclical use at design pressures of 0-10,000 psig. More preferably, the pressure vessel is tested using cyclical pressure tests at 1.25-1.5 times design pressure in order to meet safety standards.
Using this novel autofrettage process, the user of the vessel produced thereby can be confident that the vessel has been subjected to at least 1.5 times the service pressure, even with the higher service pressures now sought in the industry.